Lithium forms monoxide while sodium forms peroxide in the presence of excess oxygen-why?

Explanation

Larger cations can be stabilised by larger anions because if both the ions are comparable in size, the coordination number will be high and this gives rise to a high lattice enthalpy. Lithium ion, `Li^(+)` as well as oxide ion, `O^(2-)` , have small ionic radii and high charge densities. Hence these small ions combine together and form a very stable lattice of lithium monoxide `(Li_2O)`. Similarly, formation of sodium peroxide `(Na_2O_2)` can be explained on the basis of the stable lattice formed by the packing ofr elatively large `Na^(+)` ion and peroxide ion `O_2^(2-)`.